A known method for providing each cylinder group with a rich or lean air-fuel ratio is used to perform sulfur poisoning recovery of a NOx catalyst. A known device (as disclosed, for example, in the following Patent Document 1) varies the fuel injection amount for each cylinder group in order to provide each cylinder group with a rich or lean air-fuel ratio. Another known device (as disclosed, for example, by the following Patent Document 2) varies the fuel injection amount and intake air amount for each cylinder group in order to provide each cylinder group with a rich or lean air-fuel ratio.
[Patent Document 1]
Japanese Patent Laid-Open No. 2003-97259
[Patent Document 2]
Japanese Patent Laid-Open No. 2001-329872
[Patent Document 3]
Japanese Patent Laid-Open No. 2000-352310